Negative electrode for lithium secondary battery and method of manufacturing the same

ABSTRACT

Provided are a negative electrode for a lithium secondary battery and a method of manufacturing the same. The negative electrode for a lithium secondary battery according to an embodiment of the present invention includes a negative electrode active material including: a silicon oxide, lithium, and sodium or potassium, wherein in ICP analysis of a negative electrode active material layer including the negative electrode active material, contents of elements in the negative electrode active material layer satisfy the following Relations (1) and (2): 
       300≤10 6 *A/(B 2 +C 2 )≤12.0*10 6   (1)
 
       800≤A≤140,000  (2)
         wherein A is a Li content in ppm, B is a Na content in ppm, and C is a K content in ppm, based on the total weight of the ICP-analyzed negative electrode active material layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to Korean PatentApplication No. 10-2022-0076581, filed on Jun. 23, 2022, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The following disclosure relates generally to a negative electrode for alithium secondary battery and a method for manufacturing the same.

BACKGROUND

In recent years, demand for environmentally friendly technologies forsolving the global warming problem is rapidly increasing. In particular,as demand for electric vehicles and energy storage systems (ESS)increases, demand for lithium secondary batteries is also exploding.

A conventional lithium secondary battery generally uses a carbon(C)-based negative electrode material such as natural graphite andartificial graphite, but the energy density of a battery using graphiteis low due to the low theoretical capacity of graphite of 372 mAh/g.Therefore, studies on a new negative electrode material for improvingthe low energy density are in progress.

As a solution for increasing the energy density, a silicon (Si)-basednegative electrode material having a high theoretical capacity of 3580mAh/g has been proposed. However, the silicon-based negative electrodematerial has poor battery life characteristics due to a large volumeexpansion (˜400%) in the process of repeated charging and discharging.

As a method for solving the volume expansion issue of the silicon-basednegative electrode material, a silicon oxide-based negative electrodematerial having a volume expansion rate lower than Si has beendeveloped. The silicon oxide-based negative electrode material showsbetter life characteristics than the silicon-based negative electrodematerial, due to its low volume expansion rate. However, a battery usingthe silicon oxide-based negative electrode material has poor initialcoulombic efficiency (ICE) due to the formation of an irreversible phaseat the beginning of operation, and has poor industrial usability due tothe poor initial coulombic efficiency. In addition, the siliconoxide-based negative electrode material has low electrical conductivityand a non-uniform current distribution of the surface, so that the lifecharacteristics of a battery using the silicon oxide-based negativeelectrode material are generally poor.

SUMMARY

An embodiment of the present invention is directed to providing anegative electrode for a lithium secondary battery exhibiting improvedinitial coulombic efficiency (also referred to hereinafter simply asinitial efficiency) and capacity and improved battery lifecharacteristics, and a method of manufacturing the same.

Another embodiment of the present invention is directed to furtherimproving life characteristics of a lithium secondary battery bysecuring a uniform current distribution on a surface of a negativeelectrode active material in a prelithiated negative electrode.

In one general aspect, a negative electrode for a lithium secondarybattery includes a negative electrode active material including: asilicon oxide; lithium; and sodium or potassium, wherein in ICP(inductively coupled plasma spectrometer) analysis of a negativeelectrode active material layer including the negative electrode activematerial, contents of elements in the negative electrode active materiallayer satisfy the following Relations (1) and (2):

300≤10⁶*A/(B²+C²)≤12.0*10⁶  (1)

800≤A≤140,000  (2)

wherein A is a Li content in ppm (parts per million), B is a Na (sodium)content in ppm, and C is a K (potassium) content in ppm, based on thetotal weight of the ICP-analyzed negative electrode active materiallayer.

In addition, in the negative electrode for a lithium secondary batteryaccording to an embodiment of the present invention, in the ICP analysisof the negative electrode active material layer, the contents ofelements in the negative electrode active material layer may furthersatisfy the following Relation (3) or (4):

50≤B≤20,000  (3)

15≤C≤10,000  (4)

wherein B is a Na content in ppm, and C is a K content in ppm, based onthe total weight of the ICP-analyzed negative electrode active materiallayer.

In addition, according to an embodiment of the present invention, inRelation (3), 200≤B≤20,000 may be satisfied, or in Relation (4),30≤C≤10,000 may be satisfied.

In addition, in the negative electrode for a lithium secondary batteryaccording to an embodiment of the present invention, in the ICP analysisof the negative electrode active material layer, the contents ofelements in the negative electrode active material layer may furthersatisfy the following Relation (5):

25≤B+C  (5)

wherein B is a Na content in ppm, and C is a K content in ppm, based onthe total weight of the ICP-analyzed negative electrode active materiallayer.

In addition, according to an embodiment of the present invention, thenegative electrode active material may include a lithium silicaterepresented by the following Chemical Formula 1:

Li_(x)Si_(y)O_(z)  [Chemical Formula 1]

wherein 1≤x≤6, 1≤y≤4, and 0<z≤7.

In addition, the negative electrode for a lithium secondary batteryaccording to an embodiment of the present invention may further includeartificial graphite.

In addition, the negative electrode for a lithium secondary batteryaccording to an embodiment of the present invention may further includesingle-walled carbon nanotubes.

In another general aspect, a method of manufacturing a negativeelectrode for a lithium secondary battery includes: a1) performingpre-lithiation by mixing a silicon-based material including a siliconoxide and a lithium precursor and heat treating the mixture to dope thesilicon-based material with lithium; a2) stirring a solution ordispersion including the silicon-based material doped with lithium and asodium precursor or a potassium precursor; and a3) heat treating theproduct of the process a2), thereby preparing a negative electrodeactive material doped with sodium or potassium.

In addition, according to an embodiment of the present invention, in theprocess a1), the silicon-based material and the lithium precursor may bemixed so that a mole ratio of Li/Si is 0.3 to 1.2.

In addition, according to an embodiment of the present invention, theprocess a2) may be the stirring of the solution or dispersion includingthe silicon-based material doped with lithium and the sodium precursoror the potassium precursor so that a Na/Si mole ratio is more than 0 and0.05 or less or a K/Si mole ratio is more than 0 and 0.08 or less.

In addition, according to an embodiment of the present invention, in theprocess a2), a stirring speed may be 100 to 3000 rpm.

In addition, according to an embodiment of the present invention, in theprocess a2), a temperature during stirring may be 15 to 80° C.

In addition, according to an embodiment of the present invention, theheat treating in the process a3) may be performed at 200 to 1000° C.

In addition, according to an embodiment of the present invention, thesodium precursor may include one of a Na metal; a Na oxide; a Nacompound or Na oxide containing one or more of F, Cl, Br, I, C, N, P, S,and H; and a Na composite oxide containing one or more metals of Li, Ti,V, Cr, Mn, Fe, Co, and Ni, or a combination thereof.

In addition, according to an embodiment of the present invention, thepotassium precursor may include one of a K metal; a K oxide; a Kcompound or K oxide containing one or more of F, Cl, Br, I, C, N, P, S,and H; and a K composite oxide containing one or more metals of Li, Ti,V, Cr, Mn, Fe, Co, and Ni, or a combination thereof.

In addition, according to an embodiment of the present invention, thesilicon-based material may include SiO_(x) (0<x≤2); and one or more ofSi, a Si-containing alloy, and a Si/C composite.

In another general aspect, a method of manufacturing a negativeelectrode for a lithium secondary battery includes: b1) stirring asolution or dispersion including a silicon-based material including asilicon oxide, a lithium precursor, and a sodium precursor or apotassium precursor; and b2) heat treating the product of the processb1), thereby preparing a negative electrode active material doped withsodium or potassium.

In addition, according to an embodiment of the present invention, theprocess b1) may be the stirring of the solution or dispersion includingthe silicon-based material and the sodium precursor or the potassiumprecursor so that a Na/Si mole ratio is more than 0 and 0.05 or less ora K/Si mole ratio is more than 0 and 0.08 or less.

In addition, according to an embodiment of the present invention, theheat treating in the process b2) may be performed at 200 to 1000° C.

In still another general aspect, a lithium secondary battery includesthe negative electrode according to one embodiment of the embodimentsdescribed above.

Other features and aspects will be apparent from the following detaileddescription, the drawings, and the claims.

DETAILED DESCRIPTION OF EMBODIMENTS

Advantages and features of the present invention and methods to achievethem will be elucidated from embodiments described below in detail withreference to the accompanying drawings. However, the present inventionis not limited to embodiments disclosed below, but will be implementedin various forms. The embodiments of the present invention provide athorough disclosure of the present invention so that those skilled inthe art can easily understand the the present invention. However, thescope of the present invention will be defined by the scope of theappended claims. Detailed description for carrying out the presentinvention will be provided with reference to the accompanying drawingsbelow. Same reference numbers in different drawings indicate similar oridentical elements. The phrase “and/or” includes each of and allcombinations of one or more of mentioned items.

Unless otherwise defined herein, all terms used in the specification(including technical and scientific terms) may have the meaning that iscommonly understood by those skilled in the art. Throughout the presentspecification, unless explicitly described to the contrary, “comprising”any elements will be understood to imply further inclusion of otherelements rather than the exclusion of any other elements. In addition,unless explicitly described to the contrary, a singular form includes aplural form herein.

In the present specification, it will be understood that when an elementsuch as a layer, film, region, or substrate is referred to as being “on”or “above” another element, it can be directly on the other element orintervening elements may also be present.

In the present specification, an average particle size may refer to D50,and D50 refers to a diameter of a particle with a cumulative volume of50% when cumulated from the smallest particle in measurement of aparticle size distribution by a laser scattering method. Here, for D50,the particle size distribution may be measured by collecting a sampleaccording to the KS A ISO 13320-1 standard and using Mastersizer 3000from Malvern Panalytical Ltd. Specifically, a volume density may bemeasured after dispersion is performed using ethanol as a solvent, and,if necessary, using an ultrasonic disperser.

According to an embodiment of the present invention, in order tocomplement low initial efficiency (ICE) at the beginning of batteryoperation to which a silicon oxide-based negative electrode material isapplied, a pre-lithiation process of a negative electrode activematerial including a silicon oxide may be performed. The lithiumsilicate produced by the pre-lithiation process may complement the lowinitial efficiency of a battery.

According to an embodiment of the present invention, in order to securea uniform current distribution on the surface of the negative electrodeactive material to improve life characteristics, sodium or potassium maybe single doped or co-doped after or during the pre-lithiation process.

According to an embodiment of the present invention, a negativeelectrode for a lithium secondary battery including a negative electrodeactive material including: a silicon oxide, lithium, and sodium orpotassium may be provided, wherein in ICP analysis of a negativeelectrode active material layer including the negative electrode activematerial, contents of elements in the negative electrode active materiallayer satisfy the following Relations (1) and (2):

300≤10⁶*A/(B²+C²)≤12.0*10⁶  (1)

800≤A≤140,000  (2)

wherein A is a Li content in ppm, B is a Na content in ppm, and C is a Kcontent in ppm, based on the total weight of the ICP-analyzed negativeelectrode active material layer.

According to an embodiment, the ICP analysis of the negative electrodeactive material layer may be performed on a negative electrode extractedfrom a half battery which is manufactured by disposing a negativeelectrode to be analyzed, a lithium metal as a counter electrode, and aPE separator between the negative electrode and the counter electrodeand injecting an electrolyte solution to manufacture a CR2016 type coincell, and pausing the assembled coin cell at room temperature for 3 to24 hours. Here, the electrolyte solution injected into the coin cell maybe obtained by mixing 1.0 M LiPF=₆ as a lithium salt with an organicsolvent (EC:EMC=30:70 vol %) and mixing 2 to 5 vol % of fluoroethylenecarbonate (FEC) as an electrolyte additive therewith. The half batterymanufactured was charged at a constant current at room temperature (25°C.) until the voltage reached 0.01 V (vs. Li/Li⁺) at a current of 0.1 Crate, and then was charged with a constant voltage by cut-off at acurrent of 0.01 C rate while maintaining 0.01 V in a constant voltagemode. The battery was discharged at a constant current of 0.1 C rateuntil the voltage reached 1.5 V (vs. Li/Li⁺). One charge and dischargecycle was performed under the charge and discharge conditions, and thendisassembly was performed to obtain a negative electrode. Next, thenegative electrode obtained by disassembly was washed with an organicsolvent such as dimethyl carbonate (DMC) several times, and negativeelectrode active material layer powder was recovered by scrapping offthe powder so that a current collector is not included.

A method of measuring the Li content (A), the Na content (B), and Kcontent (C) using the negative electrode active material layer powderrecovered above may be the following:

-   -   [1] adding 0.01 to 0.05 g of the recovered negative electrode        active material layer powder to a 50 mL PP tube;    -   [2] adding nitric acid to the PP tube and then adding a        hydrofluoric acid thereto until brown fume does not occur;    -   [3] heating the PP tube with a heating block and drying the tube        to remove a hydrofluoric component;    -   [4] adding nitric acid and hydrogen peroxide to the PP tube and        then heating the PP tube with a heating block for redissolving;    -   [5] cooling the resulting product to room temperature, diluting        it with ultrapure water, and filtering it for removing insoluble        components to prepare a sample; and    -   [6] performing ICP analysis on the prepared sample to measure        the Li content (A), the Na content (B), and the K content (C)        (A, B, and C are the weights (ppm) of Li, Na, and K included        based on the total weight of the negative electrode active        material layer (powder) to be measured, respectively).

Here, the ICP analysis may be performed using Optima 8300DV ICPspectrometer available from Perkin Elmer.

The negative electrode to be analyzed may be a freshly manufacturedelectrode, and may be obtained by disassembling a finished battery or abattery purchased on the market. The finished battery or the batterypurchased on the market may be previously subjected to 5 cycles or lessof charge and discharge during the manufacturing process of the battery,for example, a formation process or the like. However, since a change inthe resulting value of the ICP analysis of the negative electrode activematerial layer is very small by performing 5 cycles or less of chargeand discharge, the negative electrode obtained by disassembling afinished battery or a battery purchased on the market was manufacturedinto a half battery under the same conditions as described above, thehalf battery was discharged to 1.5 V (vs. Li/Li⁺), the negativeelectrode was disassembled, and ICP analysis may be performed on thedisassembled negative electrode by the same method as described above.

According to the present invention, by performing the ICP analysis ofthe negative electrode active material layer according to an embodimentdescribed above, the contribution of sodium or potassium doped into aprelithiated negative electrode material to the uniform currentdistribution on the surface of the negative electrode active materialmay be quantitatively evaluated.

Hereinafter, the reasons for defining Relations (1) and (2) will bedescribed, respectively.

A sodium or potassium cation allows easy approach of electrons, but hasvery low reactivity with a negative electrode active material containinga silicon oxide. In view of this, Relation (1) is a parameter fordecreasing local deterioration of an electrode due to repeated cycles tosecure an effect of improving life characteristics, by securing auniform current distribution on the surface of a negative electrodeactive material by single doping or co-doping of sodium or potassium.The resulting value of Relation (1) may be derived by substituting thenumerical values of the Li content in ppm, the Na content in ppm, and Kcontent in ppm without a unit into Relation (1).

When Relation (1) is less than the lower limit 300, the content of dopedsodium or potassium is excessive as compared with the Li ion, and thus,a smooth reaction between Li ions and electrons is hindered, so that thelife characteristics may be rather deteriorated, which is not preferred.From the point of view of the smooth reaction between Li ions andelectrons, the lower limit of Relation (1) may be, for example, 400 ormore, 500 or more, 600 or more, or 700 or more.

However, when the value is more than the upper limit of Relation (1) of12.0*10⁶, the content of doped sodium or potassium is too small ascompared with the Li ion, and thus, the effect of improving the lifecharacteristics by securing a uniform current distribution on thesurface of the negative electrode active material by adding sodium orpotassium may not be sufficiently secured. From the point of view ofsecuring the uniform current distribution on the surface of the negativeelectrode active material by single doping or co-doping of sodium orpotassium, the upper limit of Relation (1) may be preferably, forexample, 11.0*10⁶ or less, 10.0*10⁶ or less, or 9.0*10⁶.

Relation (2) is a parameter for securing sufficient initial batteryefficiency by pre-lithiation. When the value is less than the lowerlimit of Relation (2) of 800, sufficient initial efficiency may not besecured by pre-lithiation. When the value is more than the upper limitof Relation (2) of 140,000, a lithium content is excessive, so that itis difficult to store the prepared negative electrode active material inthe air without deterioration, and it is difficult to secure slurrystability due to a reaction with water in the preparation of the slurry.In addition, a molecular binding of a binder is broken, and thus,control to a certain level of viscosity required for coating may not beallowed, making electrode coating difficult, and it is difficult tosecure a slurry for a long time due to gas generation. In addition,since an excessive amount of lithium is used as compared with an optimallithium content, an expensive lithium raw material should be used in alarge amount in the preparation of an active material, which may causecosts to rise. In addition, in particular, when an excessive amount oflithium is doped, the excessive amount of a lithium compound surroundsthe negative electrode active material so that sodium or potassium maynot react with or be doped into the negative electrode active materialdirectly, and during preparation of a slurry, an excessive amount oflithium compound on the surface of the negative electrode activematerial is dissolved in moisture so that sodium or potassium is removedtogether, and thus, sodium or potassium may not be sufficiently doped.

From the foregoing point of view, in a preferred embodiment, A may be,for example, 800 or more, 1,000 or more, 2,000 or more, 3,000 or moreand 140,000 or less, 120,000 or less, 100,000 or less, 80,000 or less,50,000 or less, or between these numerical values. In an embodiment, Amay be 800 to 140,000, 2,000 to 100,000, and 3,000 to 80,000.

According to an embodiment of the present invention, by satisfyingRelations (1) and (2), the sufficient initial efficiency of a batterymay be secured by pre-lithiation, and the uniform current distributionon the surface of the negative electrode active material is secured bysingle doping or co-doping of sodium or potassium to improve the lifecharacteristics of a battery.

When Relations (1) and (2) are satisfied, the life characteristics maybe sufficiently improved, but according to a preferred embodiment of thepresent invention, it is preferred that in the ICP analysis of thenegative electrode active material layer, the contents of elements inthe negative electrode active material layer further satisfy thefollowing Relation (3) or (4), since the effects described above may bebetter secured:

50≤B≤20,000  (3)

15≤C≤10,000  (4)

wherein B is a Na content in ppm, and C is a K content in ppm, based onthe total weight of the ICP-analyzed negative electrode active materiallayer.

Hereinafter, the reasons for defining Relations (3) and (4) will bedescribed.

Relations (3) and (4) are parameters for securing an effect of improvinglife characteristics by securing a uniform current distribution bysodium doping in Relation (3) and potassium doping in Relation (4).According to an embodiment of the present invention, when Relation (3)or (4) is further satisfied in addition to Relations (1) and (2), anappropriate amount of sodium or potassium for further improving the lifecharacteristics by securing a uniform current distribution on thesurface of the negative electrode active material may be doped, which isthus preferred.

From the foregoing point of view, in an embodiment, B may be, forexample, 50 or more, 100 or more, 200 or more, 300 or more and 20,000 orless, 150,000 or less, 10,000 or less, or between these numericalvalues. In an embodiment, B may be 50 to 20,000, 100 to 20,000, and is200 to 20,000 or 300 to 20,000.

From the foregoing point of view, in an embodiment, C may be, forexample, 15 or more, 30 or more, 40 or more and 10,000 or less, 8,000 orless, 5,000 or less, 3,000 or less, or between these numerical values.In an embodiment, C may be 15 to 10,000, preferably 30 to 10,000, andmore preferably 40 to 10,000.

When Relations (1) and (2) are satisfied, the life characteristics maybe sufficiently improved, but according to a preferred embodiment of thepresent invention, it is preferred that in the ICP analysis of thenegative electrode active material layer, the contents of elements inthe negative electrode active material layer further satisfy thefollowing Relation (5), since the effects described above may be betterprovided:

25≤B+C  (5)

wherein B is a Na content in ppm, and C is a K content in ppm, based onthe total weight of the ICP-analyzed negative electrode active materiallayer.

Hereinafter, the reason for defining Relation (5) will be described.

Relation (5) is a parameter for securing an effect of improving lifecharacteristics by securing a uniform current distribution, consideringthe case of single doping or co-doping of sodium or potassium. Accordingto an embodiment of the present invention, when Relation (5) is furthersatisfied in addition to Relations (1) and (2), an appropriate amount ofsodium or potassium for further improving the life characteristics bysecuring a uniform current distribution on the surface of the negativeelectrode active material may be doped.

From the foregoing point of view, in a preferred embodiment, B+C may be,for example, 25 or more, 30 or more, or more, 50 or more, 60 or more, 70or more, 80 or more and 10,000 or less, 8,000 or less, 7,000 or less, orbetween these numerical values. In an embodiment, B+C may be 25 to to10,000, 40 to 10,000, 50 to 10,000, 60 to and 70 to 10,000 or 80 to10,000.

According to an embodiment of the present invention, the negativeelectrode may include a silicon oxide, lithium, and sodium or potassium.

In addition, the negative electrode active material may include SiO_(x)(0<x≤2); and one or more of Si, a Si-containing alloy, and a Si/Ccomposite. The Si-containing alloy may be, for example, a Si-Q alloy. Qis an element selected from the group consisting of alkali metals,alkaline earth metals, group 13 elements, group 14 elements other thanSi, group 15 elements, group 16 elements, transition metals, rare earthelements, and combinations thereof. The element Q may be, for example,selected from the group consisting of Li, Mg, Na, K, Ca, Sr, Ba, Ra, Sc,Y, Ti, Zr, Hf, Rf, V, Nb, Ta, db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru,Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P,As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

The negative electrode for a lithium secondary battery according to anembodiment of the present invention performs a pre-lithiation process interms of further improving initial efficiency and life characteristics,and the negative electrode active material may include lithium. Lithiummay be doped into or chemically bonded to the negative electrode activematerial.

According to an embodiment, the negative electrode active material mayfurther include a lithium silicate represented by the following ChemicalFormula 1:

Li_(x)Si_(y)O_(z)  [Chemical Formula 1]

wherein 1≤x≤6, 1≤y≤4, and 0<z≤7.

The negative electrode for a lithium secondary battery according to anembodiment of the present invention includes a current collector, and anegative electrode active material layer which is prepared on thecurrent collector and includes the negative electrode active materialand a binder.

According to an embodiment of the present invention, the currentcollector may be selected from the group consisting of a copper foil, anickel foil, a stainless steel foil, a titanium foil, a nickel foam, acopper foam, a polymer substrate coated with a conductive metal and acombination thereof. However, the current collector is not limitedthereto.

According to an embodiment of the present invention, the negativeelectrode active material may further include a material capable ofreversibly inserting/desorbing a lithium ion, a lithium metal, an alloyof lithium metal, a material capable of being doped and de-doped withlithium, or a transition metal oxide.

An example of the material capable of reversibly inserting/desorbing alithium ion may include a carbon material, that is, a carbon-basednegative electrode active material which is commonly used in the lithiumsecondary battery. An example of the carbon-based negative electrodeactive material may include crystalline carbon, amorphous carbon, or acombination thereof. A crystalline carbon may include, for example,graphite such as amorphous, plate-like, flake-like, spherical, orfibrous natural graphite or artificial graphite. An example of amorphouscarbon may include soft carbon, hard carbon, a mesophase pitch carbide,calcined coke, and the like.

The alloy of lithium metal may be, for example, an alloy of lithium witha metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg,Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn.

The material capable of doping or de-doping with lithium may be asilicon-based material, for example, Si, SiO_(x) (0<x≤2), a Si-Q alloy(Q is an element selected from the group consisting of alkali metals,alkaline earth metals, Group 13 elements, Group 14 elements, Group 15elements, Group 16 elements, transition metals, rare-earth elements, andcombinations thereof, but is not Si), a Si-carbon composite, Sn, SnO₂, aSn—R alloy (R is an element selected from the group consisting of alkalimetals, alkaline earth metals, Group 13 elements, Group 14 elements,Group 15 elements, Group 16 elements, transition metals, rare-earthelements, and combinations thereof, but is not Si), a Sn-carboncomposite, and the like, and also, a mixture of at least one thereof andSiO₂ may be used. The elements Q and R may be, for example, selectedfrom the group consisting of Na, K, Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr,Hf, Rf, V, Nb, Ta, db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs,Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb,Bi, S, Se, Te, Po, and combinations thereof.

The transition metal oxides may be, for example, a lithium titaniumoxide.

According to an embodiment of the present invention, the binder servesto adhere the negative electrode active material particles to each otherwell, and also serves to adhere the negative electrode active materialto the current collector well. As the binder, all binders known in theart may be used, but for example, it may be a water-based binder, and anon-limiting example of the water-based binder may includepolyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,ethylene-propylene-diene polymer (EPDP), sulfonated-EPDM,styrene-butadiene rubber (SBR), fluorine rubber, various copolymersthereof, and the like. For example, the binder may include one ofcarboxyl methyl cellulose (CMC), styrene-butadiene rubber (SBR), and amixture thereof.

The negative electrode active material layer according to an embodimentmay further selectively include a conductive material. The conductivematerial is used for imparting conductivity to an electrode, and anymaterial having conductivity without causing a chemical change to abattery may be used without limitation. An example of the conductivematerial may include a carbon-based material such as natural graphite,artificial graphite, carbon black, acetylene black, ketjen black, carbonfiber, single-walled carbon nanotubes (SW-CNT), and multi-walled carbonnanotubes (MW-CNT); a metal-based material such as metal powder or metalfiber of copper, nickel, aluminum, silver, and the like; a conductivepolymer such as a polyphenylene derivative; or a mixture thereof.

The contents of the binder and the conductive material in the negativeelectrode active material layer may be, for example, 0.1 to 10 wt %, forexample, 0.1 to 5 wt % with respect to the total weight of the negativeelectrode active material layer. However, the contents of the binder andthe conductive material are not limited to the example described above.

In the negative electrode for a lithium secondary battery according tothe present invention described above, according to an embodiment, thenegative electrode active material may be prepared by mixing a negativeelectrode active material precursor including a silicon oxide with asolution or dispersion prepared by dissolving or dispersing a sodiumprecursor or potassium precursor as a doping raw material in awater-based or non-water-based solution or dispersion medium, andperforming a heat treatment, but the present invention is notparticularly limited thereto, and the negative electrode may bemanufactured by various manufacturing methods without limitation, aslong as Relations (1) and (2) are satisfied, or, Relation (3), (4), or(5) is further satisfied. The water-based or non-water-based solvent ordispersion medium may be all materials known in the art, and is notparticularly limited.

In particular, in the embodiment, when a solution or dispersion, inwhich a sodium precursor or a potassium precursor is dissolved ordispersed, and a negative electrode active material including a siliconoxide are stirred, sodium or potassium may be uniformly doped in thenegative electrode active material to secure a uniform currentdistribution, which is thus preferred. Meanwhile, when an alloy or thelike including sodium or potassium is brought into simple pressurecontact or contact with the surface of a negative electrode plate onwhich a silicon-based material thin film is prepared, and then iselectrochemically inserted or doped using thermal diffusion, sodium orpotassium may not be uniformly doped, and in this case, a uniformcurrent distribution may not be secured and life characteristics to bedesired may not be secured, which is thus not preferred.

Hereinafter, the method of manufacturing a negative electrode for alithium secondary battery according to an embodiment of the presentinvention will be described.

According to an embodiment of the present invention, a method ofmanufacturing a negative electrode for a lithium secondary batteryincluding: a1) performing pre-lithiation by mixing a silicon-basedmaterial including a silicon oxide and a lithium precursor and heattreating the mixture to dope the silicon-based material with lithium;a2) stirring a solution or dispersion including the silicon-basedmaterial doped with lithium and a sodium precursor or a potassiumprecursor; and a3) heat treating the product of the process a2), therebypreparing a negative electrode active material doped with sodium orpotassium, may be provided.

According to an embodiment of the present invention, the silicon-basedmaterial containing the silicon oxide may include SiO_(x) (0<x≤2); andone or more of Si, a Si-containing alloy, and a Si/C composite. TheSi-containing alloy may be, for example, a Si-Q alloy. Q is an elementselected from the group consisting of alkali metals, alkaline earthmetals, group 13 elements, group 14 elements other than Si, group 15elements, group 16 elements, transition metals, rare earth elements, andcombinations thereof. The element Q may be, for example, selected fromthe group consisting of Li, Mg, Na, K, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr,Hf, Rf, V, Nb, Ta, db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs,Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb,Bi, S, Se, Te, Po, and combinations thereof.

According to an embodiment of the present invention, the silicon-basedmaterial may be prepared by appropriately mixing silicon powder andsilicon oxide (SiO_(x) (0<x≤2)) powder, and then performing a heattreatment at a temperature of 500 to 1600° C. for 1 to 12 hours under aninert atmosphere and reduced pressure conditions. The preparedsilicon-based material may be produced into particles by pulverization.

The lithium precursor of the process a1) may include a lithium hydride,a lithium hydroxide, a lithium oxide, a lithium carbonate, lithiumparticles, or a combination thereof. For example, the lithium precursormay include one or more of LiOH, Li, LiH, Li₂O, and Li₂CO₃.

The pre-lithiation process a1) of doping the silicon-based material withlithium may be, for example, appropriately mixing the silicon-basedmaterial and the lithium precursor and then performing a heat treatment.According to an example, the silicon-based material and the lithiumprecursor may be mixed so that a Li/Si molar ratio is 0.3 to 1.2 or 0.4to 1.0.

The heat treatment a1) may be performed at 500 to 1000° C. or 500 to700° C. for 1 to 12 hours. A lithium doping process using anelectrochemical method of oxidation reduction method may be used toeasily form lithium silicate. However, a production rate of a lithiumsilicate having a relatively significantly improved volume expansionmitigation effect among lithium silicates under the heat treatmentconditions described above is higher. For example, the production rateof Li₂SiO₃ having a relatively significantly improved volume expansionmitigation effect among lithium silicates is higher when the heattreatment conditions described above is applied than when anelectrochemical method or an oxidation reduction method is used, whichis thus advantageous for improving the battery life characteristics.

According to an embodiment of the present invention, the silicon-basedmaterial may be doped with lithium by the pre-lithiation process a1)described above. According to an example, lithium silicate may beprepared in at least a part of the silicon-based material.

After the pre-lithiation process a1) described above, according to anembodiment of the present invention, a process a2) of stirring asolution or dispersion including the silicon-based material doped withlithium and a sodium precursor or a potassium precursor may beperformed. According to an example, the solution or dispersion may beprepared by dispersing or dissolving a sodium precursor or a potassiumprecursor as a doping raw material in a water-based or non-water-basedsolvent or dispersion medium, and may be heat treated after mixing itwith the silicon-based material doped with lithium. The water-based ornon-water-based solvent or dispersion medium may be all materials knownin the art, and is not particularly limited.

According to an embodiment of the present invention, the sodiumprecursor may include one of a Na metal; a Na oxide; a Na compound or Naoxide containing one or more of F, Cl, Br, I, C, N, P, S, and H; and aNa composite oxide containing one or more metals of Li, Ti, V, Cr, Mn,Fe, Co, and Ni, or a combination thereof. The sodium precursor may be,for example, Na, NaOH, NaNO₃, Na₃PO₄, Na₂CO₃, Na₂S, Na₂SO₄, Na₂SO₃,Na₂S₂O₈, CH₃COONa, NaF, NaCl, NaBr, NaI, HOC (COONa) (CH₂COOH)₂,Na_(x)CoO₂ (0<x≤1), Na_(x)Co_(2/3)Mn_(1/3)O₂ (0<x≤1),Na_(x)Fe_(1/2)Mn_(1/2)O₂ (0<x≤1), NaCrO₂,NaLi_(0.2)Ni_(0.25)Mn_(0.75)O_(2.35), Na_(0.44)MnO₂, NaMnO₂,Na_(0.7)VO₂, Na_(0.33)V₂O₅, Na₃V₂ (PO₄)₃, NaFePO₄,NaMn_(0.5)Fe_(0.5)PO₄, Na₃V₂ (PO₄)₃, Na₂FePO₄F, or Na₃V₂ (PO₄)₃, but isnot particularly limited thereto.

According to an embodiment of the present invention, the potassiumprecursor may include one of a K metal; a K oxide; a K compound or Koxide containing one or more of F, Cl, Br, I, C, N, P, S, and H; and a Kcomposite oxide containing one or more metals of Li, Ti, V, Cr, Mn, Fe,Co, and Ni, or a combination thereof. The potassium precursor may be,for example, K, KOH, KNO₃, KHSO₄, KHSO₃, KCN, KH₂PO₃, KH₂PO₄, KNO₃,K₃PO₄, K₂CO₃, K₂SO₄, K₂SO₃, CH₃COOK, KF, KCl, KBr, KI, KFeO₂, KCoO₂,KCrO₂, KMnO₂, KNiO₂, KNi_(1/2)Ti_(1/2)O₂, KNi_(1/2)Mn_(1/2)O₂,K_(2/3)Fe_(1/3)Mn_(2/3)O₂, KNi_(1/3) Co_(1/3)Mn_(1/3)O₂, K_(2/3)MnO₂,KMn₂O₄, K_(2/3)Ni_(1/3)Mn_(2/3)O₂, KNi_(1/2)Mn_(3/2)O₂, KFePO₄, KMnPO₄,or KCoPO₄, but is not particularly limited thereto.

Though is not particularly limited thereto, according to an embodimentof the present invention, the process a2) may be the stirring of thesilicon-based material doped with lithium; and a solution or dispersionincluding the sodium precursor or the potassium precursor so that aNa/Si molar ratio or K/Si molar ratio is in an appropriate range,thereby achieving the uniform current distribution effect by sodium orpotassium doping better, which is thus preferred.

In an embodiment, the Na/Si molar ratio may be more than 0, 0.001 ormore, 0.002 or more and 0.05 or less, 0.04 or less, 0.03 or less, orbetween the numerical values, and in a specific embodiment, the Na/Simolar ratio may be more than 0 and 0.05 or less, preferably 0.01 or moreand 0.04 or less, and more preferably 0.02 or more and 0.03 or less.

In an embodiment, the K/Si molar ratio may be more than 0, 0.001 ormore, 0.002 or more and 0.08 or less, 0.07 or less, 0.06 or less, 0.05or less, 0.04 or less, 0.03 or less or between the numerical values, andin a specific embodiment, the K/Si molar ratio may be more than 0 and0.08 or less, 0.001 or more and 0.06 or less, and 0.002 or more and 0.05or less.

The solvent or the dispersion medium of the solution or dispersionincluding the sodium precursor or the potassium precursor may be awater-based or non-water-based solvent or dispersion medium, and may beall materials known in the art. According to a non-limiting example, thesolvent or the dispersion medium may be water, alcohol, tetrahydrofuran(THF), or dimethylformamide (DMF).

Any stirring speed is sufficient as long as the sodium precursor or thepotassium precursor and the silicon-based material are uniformly mixed,and without particular limitation, for example, may be 100 to 3000 rpm,100 to 2000 rpm, 100 to 1000 rpm, preferably 300 to 3000 rpm, 300 to2000 rpm, 300 to 1000 rpm, more preferably 500 to 3000 rpm, 500 to 2000rpm, or 500 to 1000 rpm. The stirring time at this time is notparticularly limited, but for example, may be 1 minute to 3 hours or 10minutes to 3 hours.

Any temperature during stirring is sufficient as long as the sodiumprecursor or the potassium precursor and the silicon-based material areuniformly mixed, and without particular limitation, may be for example,15 to 80° C., 15 to 70° C., or 15 to 65° C.

According to an embodiment of the present invention, the heat treatmentprocess a3) may be performed in a temperature range of 200 to 1000° C.,200 to 600° C., 300 to 1000° C., 300 to 600° C., and 300 to 500° C. for1 hour to 12 hours. In addition, the heat treatment may be performed inan inert gas atmosphere including one or more of N₂, Ar, and Ne, or maybe performed under a reducing atmosphere including H₂ alone or 3-20% ofH₂ and the remainder Ar.

According to an embodiment of the present invention, the process ofdoping lithium and the process of doping sodium or potassium may beperformed in combination. According to an example, the silicon-basedmaterial, the sodium precursor or the potassium precursor, and thelithium precursor may be mixed simultaneously or successively.

According to another embodiment of the present invention in which theprocess of doping lithium and the process of doping sodium or potassiumare performed in combination, a method of manufacturing a negativeelectrode for a lithium secondary battery including: b1) stirring asolution or dispersion including a silicon-based material including asilicon oxide, a lithium precursor, and a sodium precursor or apotassium precursor; and b2) heat treating the product of the processb1), thereby preparing a negative electrode active material doped withsodium or potassium, may be provided.

In an embodiment for single doping or co-doping of sodium or potassium,the process b1) may be the stirring of the solution or dispersionincluding the silicon-based material and the sodium precursor or thepotassium precursor so that a Na/Si mole ratio is more than 0 and 0.05or less or a K/Si mole ratio is more than 0 and 0.08 or less.

In an embodiment, the Na/Si molar ratio may be more than 0, 0.001 ormore, 0.002 or more and 0.05 or less, 0.04 or less, 0.03 or less, orbetween the numerical values, and in a specific embodiment, the Na/Simolar ratio may be more than 0 and 0.05 or less, 0.01 or more and 0.04or less, and or more and 0.03 or less.

In an embodiment, the K/Si molar ratio may be more than 0, 0.001 ormore, 0.002 or more and 0.08 or less, 0.07 or less, 0.06 or less, 0.05or less, 0.04 or less, 0.03 or less or between the numerical values, andin a specific embodiment, the K/Si molar ratio may be more than 0 and0.08 or less, 0.001 or more and 0.06 or less, and 0.002 or more and 0.05or less.

Since each constituent of the solution or dispersion including thesilicon-based material, the lithium precursor, and the sodium precursorand the potassium precursor, or the sodium precursor or the potassiumprecursor is as described in the processes a1) to a3), the descriptionthereof will be omitted for convenience.

Any stirring speed is sufficient as long as the sodium precursor or thepotassium precursor and the silicon-based material are uniformly mixed,and without particular limitation, for example, may be 100 to 3000 rpm,100 to 2000 rpm, 100 to 1000 rpm, 300 to 3000 rpm, 300 to 2000 rpm, 300to 1000 rpm, 500 to 3000 rpm, 500 to 2000 rpm, or 500 to 1000 rpm. Thestirring time at this time is not particularly limited, but for example,may be 1 minute to 3 hours or 10 minutes to 3 hours.

Any temperature during stirring is sufficient as long as the sodiumprecursor or the potassium precursor and the silicon-based material areuniformly mixed, and without particular limitation, may be, for example,15 to 80° C., 15 to 70° C., or 15 to 65° C.

According to an embodiment of the present invention, the heat treatmentof process b2) may be performed in a temperature range of 200 to 1000°C., 200 to 600° C., 300 to 1000° C., 300 to 600° C., and 300 to 500° C.for 1 hour to 12 hours. In addition, the heat treatment may be performedin an inert gas atmosphere including one or more of N₂, Ar, and Ne, ormay be performed under a reducing atmosphere including H₂ alone or 3-20%of H₂ and the remainder Ar.

The processes a1) to a3) or the processes b1) and b2) described aboveare performed to prepare the negative electrode active material dopedwith sodium or potassium, and then, according to an embodiment of thepresent invention, the negative electrode active material preparedabove, the binder, the conductive material, and the like are mixed toprepare a negative electrode slurry, and the thus-prepared negativeelectrode slurry is applied on a current collector, dried, and rolled tomanufacture a negative electrode including a current collector and anegative electrode active material layer prepared on the currentcollector.

According to an embodiment of the present invention, a lithium secondarybattery including the negative electrode described above, a positiveelectrode, a separator provided between the negative electrode and thepositive electrode, and an electrolyte solution may be provided.

The positive electrode may include, for example, a current collector anda positive electrode active material layer formed by applying a positiveelectrode slurry including a positive electrode active material on thecurrent collector.

The current collector may be the negative electrode current collectordescribed above, and any known material in the art may be used, but thepresent invention is not limited thereto.

The positive electrode active material layer includes a positiveelectrode active material, and optionally, may further include a binderand a conductive material. The positive electrode active material may beany positive electrode active material known in the art, and may be, forexample, a composite oxide of a metal selected from cobalt, manganese,nickel, and a combination thereof with lithium. However, the positiveelectrode active material is not limited thereto.

The binder and the conductive material may be, for example, the negativeelectrode binder described above and the negative electrode conductivematerial, and may be a material known in the art. However, the binderand the conductive material are not limited to the example describedabove.

The separator may include, for example, glass fiber, polyester,polyethylene, polypropylene, polytetrafluoroethylene, or a combinationthereof, and may be in the form of nonwoven or woven fabric. Theseparator may be a polyolefin-based polymer separator such aspolyethylene and polypropylene, a separator coated with a compositionincluding a ceramic component or a polymer material for securing heatresistance or mechanical strength, or a separator known in the art. Theseparator may have, for example, optionally a monolayer or multilayerstructure. However, the material and the shape of the separator are notlimited to the examples.

The electrolyte solution may include an organic solvent and a lithiumsalt.

The organic solvent serves as a medium in which ions involved in theelectrochemical reaction of a battery may move. The organic solvent maybe, for example, a carbonate-based, ester-based, ether-based,ketone-based, alcohol-based, or aprotic solvent alone or in combinationof two or more, and when it is used in combination of two or more, amixing ratio may be properly adjusted depending on the batteryperformance to be desired. However, the organic solvent is not limitedto the examples described above.

The lithium salt is dissolved in the organic solvent and acts as asource of lithium ions in the battery to allow basic operation of thelithium secondary battery, and promotes movement of lithium ions betweena positive electrode and a negative electrode. The lithium salt mayinclude, for example, LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiN(SO₃C₂F₅)₂,LiN(CF₃SO₂)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂, LiAlCl₄, LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (x and y are natural numbers), LiCl, LiI, LiB(C₂O₄)₂,or a combination thereof. However, the lithium salt is not limited tothe examples described above.

A concentration of the lithium salt may be, for example, 0.1 to 2.0 M.When the lithium salt concentration is within the range, the electrolytesolution has appropriate conductivity and viscosity, and thus,significantly improved electrolyte solution performance may be shown.

The electrolyte solution according to an embodiment may further includepyridine, triethylphosphate, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphate triamide, a nitrobenzene derivative,sulfur, a quinone imine dye, N-substituted oxazolidinone,N,N-substituted imidazolidine, ethylene glycol dialkyl ether, anammofnium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, and thelike, if necessary, for improving charge and discharge characteristics,flame retardant characteristics, and the like. For example, theelectrolyte solution may further include a halogen-containing solventsuch as carbon tetrachloride and ethylene trifluoride for impartingincombustibility. For example, the electrolyte solution may furtherinclude fluoro-ethylene carbonate (FEC), propene sulfone (PRS),fluoro-propylene carbonate (FPC), and the like for improvingpreservation properties at a high temperature.

The lithium secondary battery according to an embodiment of the presentinvention may be manufactured by laminating the negative electrode, theseparator, and the positive electrode in this order to form an electrodeassembly, placing the manufactured electrode assembly in a cylindricalbattery case or an angled battery case, and then injecting anelectrolyte solution. The lithium secondary battery according to anotherembodiment may be manufactured by laminating the electrode assembly,immersing the assembly in the electrolyte solution, placing theresultant product in a battery case, and sealing the case. However, themethod of manufacturing a lithium secondary battery is not limited tothe examples described above.

As the battery case, those commonly used in the art may be adopted,there is no limitation in appearance depending on the battery use, andfor example, a cylindrical shape, an angled shape, a pouch shape, a coinshape, or the like using a can may be used.

The lithium secondary battery according to the present invention may beused in a battery cell used as a power supply of a small device, andalso may be preferably used as a battery cell in a medium or largebattery module including a plurality of battery cells. An example of themedium or large battery module may include an electric automobile, ahybrid electric automobile, a plug-in hybrid electric automobile, asystem for power storage, and the like. However, the use of the lithiumsecondary battery is not limited to the examples described above.

Hereinafter, the preferred examples and the comparative examples of thepresent invention will be described. However, the following examples areonly a preferred embodiment of the present invention, and the presentinvention is not limited thereto.

EXAMPLES Example 1

Preparation of Silicon-Based Material

A raw material in which Si and SiO_(x) (0<x≤2) were mixed was introducedto a reaction furnace and evaporated at 600° C. for 5 hours in theatmosphere having a vacuum degree of 10 Pa, and the resulting productwas deposited on a suction plate and sufficiently cooled, and then adeposit was taken out and pulverized. The pulverized silicon-basedmaterial was adjusted by sorting to obtain particles having an averageparticle diameter (D50) of about 8.0 μm.

Pre-Lithiation Process

The silicon-based material and LiH powder were mixed so that a Li/Simolar ratio was 0.4 to 1.0, thereby forming mixed powder, which washeat-treated at 700° C. for 4 to 10 hours in a nitrogen gas atmosphere.Subsequently, the heat-treated powder was recovered and pulverized in amortar, thereby doping the silicon-based material with lithium.

Sodium or Potassium Doping Process

An aqueous dispersion in which lithium-doped silicon-based material anda sodium precursor (NaNO₃) were dispersed so that a Na/Si molar ratiowas 0.025 was stirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutesto 2 hours. The resultant product from stirring was filtered and dried,heat-treated at 300 to 500° C. for 30 minutes to 2 hours under a N₂inert atmosphere, and then recovered.

Manufacture of Negative Electrode

5 to 30 wt % of the resultant product (negative electrode activematerial) from the previous process, 66 to 92 wt % of artificialgraphite, 0.05 to 0.3 wt % of single wall-CNT (SW-CNT), 1.0 to 2.0 wt %of a carboxylmethyl cellulose (CMC) binder, 1.0 to 3.0 wt % of astyrene-butadiene rubber (SBR) binder were mixed in distilled water toprepare a negative electrode slurry. The negative electrode slurry wasapplied on a Cu foil current collector, dried, and rolled to manufacturea negative electrode having a negative electrode active material layeron the current collector by a common process.

Manufacture of Half Battery

The negative electrode manufactured, a lithium metal as a counterelectrode, and a PE separator between the negative electrode and thecounter electrode were disposed, and an electrolyte solution wasinjected to manufacture a CR2016 type coin cell. The assembled coin cellwas paused at room temperature for 3 to 24 hours to manufacture a halfbattery. At this time, the electrolyte solution was obtained by mixing1.0 M LiPF 6 as a lithium salt with an organic solvent (EC:EMC=30:70 vol%) and mixing 2 to 5 vol % of FEC as an electrolyte additive.

Example 2

In Example 2, a negative electrode and a half battery were manufacturedunder the same conditions as Example 1, except that the sodium orpotassium doping process was performed under the following conditions.

An aqueous dispersion in which lithium-doped silicon-based material anda sodium precursor (NaNO₃) were dispersed so that a Na/Si molar ratiowas 0.005 was stirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutesto 2 hours. The resultant product from stirring was filtered and dried,heat-treated at 300 to 500° C. for 30 minutes to 2 hours under a N₂inert atmosphere, and then recovered.

Example 3

In Example 3, a negative electrode and a half battery were manufacturedunder the same conditions as Example 1, except that the sodium orpotassium doping process was performed under the following conditions.

An aqueous dispersion in which lithium-doped silicon-based material anda sodium precursor (NaNO₃) were dispersed so that a Na/Si molar ratiowas 0.002 was stirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutesto 2 hours. The resultant product from stirring was filtered and dried,heat-treated at 300 to 500° C. for 30 minutes to 2 hours under a N₂inert atmosphere, and then recovered.

Example 4

In Example 4, a negative electrode and a half battery were manufacturedunder the same conditions as Example 1, except that the sodium orpotassium doping process was performed under the following conditions.

An aqueous dispersion in which lithium-doped silicon-based material anda potassium precursor (NaNO₃) were dispersed so that a K/Si molar ratiowas 0.022 was stirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutesto 2 hours. The resultant product from stirring was filtered and dried,heat-treated at 300 to 500° C. for 30 minutes to 2 hours under a N₂inert atmosphere, and then recovered.

Example 5

In Example 5, a negative electrode and a half battery were manufacturedunder the same conditions as Example 1, except that the sodium orpotassium doping process was performed under the following conditions.

An aqueous dispersion in which lithium-doped silicon-based material anda potassium precursor (NaNO₃) were dispersed so that a K/Si molar ratiowas 0.004 was stirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutesto 2 hours. The resultant product from stirring was filtered and dried,heat-treated at 300 to 500° C. for 30 minutes to 2 hours under a N₂inert atmosphere, and then recovered.

Example 6

In Example 6, a negative electrode and a half battery were manufacturedunder the same conditions as Example 1, except that the sodium orpotassium doping process was performed under the following conditions.

An aqueous dispersion in which lithium-doped silicon-based material anda potassium precursor (NaNO₃) were dispersed so that a K/Si molar ratiowas 0.002 was stirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutesto 2 hours. The resultant product from stirring was filtered and dried,heat-treated at 300 to 500° C. for 30 minutes to 2 hours under a N₂inert atmosphere, and then recovered.

Example 7

In Example 7, a negative electrode and a half battery were manufacturedunder the same conditions as Example 1, except that the sodium orpotassium doping process was performed under the following conditions.

An aqueous dispersion in which a lithium-doped silicon-based material, asodium precursor (NaNO₃), and a potassium precursor (KNO₃) weredispersed so that a Na/Si molar ratio was 0.013 and a K/Si molar ratiowas 0.011 was stirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutesto 2 hours. The resultant product from stirring was filtered and dried,heat-treated at 300 to 500° C. for 30 minutes to 2 hours under a N₂inert atmosphere, and then recovered.

Comparative Example 1

In Comparative Example 1, the pre-lithiation process and the sodium orpotassium doping process of Example 1 were not performed. Other thanthat, a negative electrode and a half battery were manufactured underthe same conditions.

Comparative Example 2

In Comparative Example 1, the sodium or potassium doping process ofExample 1 was not performed. Other than that, a negative electrode and ahalf battery were manufactured under the same conditions.

Comparative Example 3

In Comparative Example 3, a negative electrode and a half battery weremanufactured under the same conditions as in Example 1, except that thepre-lithiation process of Example 1 was not performed (no lithiumdoped), and the sodium or potassium doping process was performed underthe following conditions.

An aqueous dispersion in which a silicon-based material, a sodiumprecursor (NaNO₃), and a potassium precursor (KNO₃) were dispersed sothat a Na/Si molar ratio was 0.013 and a K/Si molar ratio was 0.011 wasstirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutes to 2 hours.The resultant product from stirring was filtered and dried, heat-treatedat 300 to 500° C. for 30 minutes to 2 hours under a N₂ inert atmosphere,and then recovered.

Comparative Example 4

In Comparative Example 4, a negative electrode and a half battery weremanufactured under the same conditions as Example 1, except that thesodium or potassium doping process was performed under the followingconditions.

An aqueous dispersion in which lithium-doped silicon-based material anda sodium precursor (NaNO₃) were dispersed so that a Na/Si molar ratiowas 0.052 was stirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutesto 2 hours. The resultant product from stirring was filtered and dried,heat-treated at 300 to 500° C. for 30 minutes to 2 hours under a N₂inert atmosphere, and then recovered.

Comparative Example 5

In Comparative Example 5, in the pre-lithiation process, lithium wasexcessively doped by adding LiH so that a Li/Si molar ratio was 2.0 ormore. Other than that, a negative electrode and a half battery weremanufactured under the same conditions.

Comparative Example 6

In Comparative Example 6, a negative electrode and a half battery weremanufactured under the same conditions as Example 1, except that thesodium or potassium doping process was performed under the followingconditions.

An aqueous dispersion in which lithium-doped silicon-based material anda potassium precursor (KNO₃) were dispersed so that a K/Si molar ratiowas 0.087 was stirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutesto 2 hours. The resultant product from stirring was filtered and dried,heat-treated at 300 to 500° C. for 30 minutes to 2 hours under a N₂inert atmosphere, and then recovered.

Comparative Example 7

In Comparative Example 7, a negative electrode and a half battery weremanufactured under the same conditions as Example 1, except that thesodium or potassium doping process was performed under the followingconditions.

An aqueous dispersion in which a lithium-doped silicon-based material,sodium precursor (NaNO₃), and a potassium precursor (KNO₃) weredispersed so that a Na/Si molar ratio was 0.052 and a K/Si molar ratiowas 0.087 was stirred at 300 to 1000 rpm at 20 to 60° C. for 30 minutesto 2 hours. The resultant product from stirring was filtered and dried,heat-treated at 300 to 500° C. for 30 minutes to 2 hours under a N₂inert atmosphere, and then recovered.

In order to analyze the battery properties of each of the examples andthe comparative examples, ICP analysis of the negative electrode activematerial layer was performed, a discharge capacity, initial efficiency,and a capacity retention rate were measured, and the results are shownin the following Table 1.

The ICP analysis of the negative electrode active material layer wasperformed as follows. The half battery manufactured was charged at aconstant current at room temperature (25° C.) until the voltage reached0.01 V (vs. Li/Li⁺) at a current of 0.1 C rate, and then was chargedwith a constant voltage by cut-off at a current of 0.01 C rate whilemaintaining 0.01 V in a constant voltage mode. The battery wasdischarged at a constant current of 0.1 C rate until the voltage reached1.5 V (vs. Li/Li⁺). One charge and discharge cycle was performed underthe charge and discharge conditions, and then disassembly was performedto obtain a negative electrode. Next, the disassembled negativeelectrode was washed with an organic solvent such as dimethyl carbonate(DMC) several times, and negative electrode active material layer powderwas recovered by scrapping off the powder so that a current collectorwas not included.

A method of measuring the Li content (A), the Na content (B), and Kcontent (C) using the negative electrode active material layer powderrecovered above followed the following:

-   -   [1] adding 0.01 to 0.05 g of the recovered negative electrode        active material layer powder to a 50 mL PP tube;    -   [2] adding a nitric acid to the PP tube and then adding a        hydrofluoric acid thereto until brown fume does not occur;    -   [3] heating the PP tube with a heating block and drying the tube        to remove a hydrofluoric component;    -   [4] adding a nitric acid and hydrogen peroxide to the PP tube        and then heating the PP tube with a heating block for        redissolving;    -   [5] cooling the resulting product to room temperature, diluting        it with ultrapure water, and filtering it for removing insoluble        components to prepare a sample; and    -   [6] performing ICP analysis on the prepared sample to measure        the Li content (A), the Na content (B), and the K content (C)        (A, B, and C are the weights (ppm) of Li, Na, and K included        based on the total weight of the negative electrode active        material layer (powder) to be measured, respectively).

The ICP analysis used Optima 8300DV available from Perkin Elmer. The ICPanalysis results were derived as weight ratios (ppm) of Li, Na, and K inthe negative electrode active material layer, based on the total weightof the ICP-analyzed negative electrode active material layer, and theresults are shown in Table 1.

The sum of Na content (B) (ppm) and the K content (C) (ppm) derived isindicated as “B+C” and shown in Table 1 together.

A value derived by substituting the Li content (A) (ppm), the Na content(B) (ppm), and the K content (C) (ppm) into “10⁶*A/(B²+C²)” of thefollowing Relation (1) is shown in Table 1 together.

300≤10⁶*A/(B²+C²)≤12.0*10⁶  (1)

wherein A is a Li content in ppm, B is a Na content in ppm, and C is a Kcontent in ppm.

The measurement for a discharge capacity (mAh/g) was performed asfollows. A TOSCAT series charging and discharging device available fromToyo system Co., LTD. was used, the half battery manufactured of each ofthe examples and the comparative examples was charged at a constantcurrent with a 0.1 C rate current until the voltage was 0.010 V (vs.Li/Li⁺) at room temperature, and then was charged with a constantvoltage by cut-off at a current of 0.01 C rate while maintaining 0.01 Vin a constant voltage mode. The battery was discharged at a constantcurrent of 0.1 C rate until the voltage reached 1.5 V (vs. Li/Li⁺).

Initial efficiency (%) was calculated as a percentage of dischargecapacity at one cycle/charge capacity at one cycle in the measurementmethod.

The life retention rate (%) was measured as follows. It was measuredusing a TOSCAT series charge and discharge available from Toyo systemCo., LTD., the discharge capacity measured after one cycle of charge anddischarge was a reference capacity, one cycle of charge and dischargewas further performed under the same conditions, an application currentwas changed to 0.5 C to perform charge and discharge, and a 10 minutepause was placed between each cycle. A percentage of the dischargecapacity after 200 cycles to the discharge capacity after one cycle ofcharge and discharge is indicated as “life retention rate (%, @200)”,and shown in Table 1.

TABLE 1 ICP analysis results of negative electrode active material layerLi Na K Life content content content Discharge Initial retention (A) (B)(C) 10⁶*A/ capacity efficiency rate (%) (ppm) (ppm) (ppm) B + C (B² +C²) (mAh/g) (%) (%, @200) Example 1 23,000 5,500 11 5,511 760 1,258 87.289.2 Example 2 46,000 490 14 504 191,431 1,317 88.1 88.4 Example 331,000 370 5 375 226,401 1,276 88.3 88.7 Example 4 4,000 27 1,050 1,0773626 1,299 87.8 87.5 Example 5 34,000 28 390 418 222,391 1,302 88.5 89.5Example 6 29,000 36 45 81 8.73*10⁶ 1,277 88.1 86.9 Example 7 26,0001,350 560 1,910 12,172 1,285 88.2 86.9 Comparative 700 22 5 27 1.38*10⁶1,110 74.1 64.2 Example 1 Comparative 25,000 15 8 23 86.5*10⁶ 1,313 87.675.8 Example 2 Comparative 300 530 2,850 3,380 36 1,201 73.2 68.1Example 3 Comparative 18,000 23,000 9 23,009 34 1170 81.1 81.6 Example 4Comparative 145,000 102 7 109 13.9*10⁶ 870 65.3 43.1 Example 5Comparative 27,000 16 13,000 13,016 160 1,184 83.1 82.2 Example 6Comparative 23,000 21,000 14,000 35,000 36 1,090 79.4 55.4 Example 7

Each of the examples and the comparative examples was evaluatedreferring to Table 1.

Referring to Table 1, Examples 1 to 7 satisfying an embodiment of thepresent invention satisfied the lithium content and Relation (1) in theICP analysis of the negative electrode active material layer defined inthe present invention, and as a result, secured a uniform currentdistribution on the surface of the negative electrode active material inthe prelithiated negative electrode to further improve batteryproperties such as life properties together with the initial efficiency.

Comparative Example 1 in which both the pre-lithiation process and thesodium or potassium doping process were not performed had an initialefficiency was 74.1% which was low, a discharge capacity of less than1,200 mAh/g, and a life retention rate of about 64.2%, and thus, hadpoor battery properties such as initial efficiency and lifecharacteristics.

Comparative Example 2 in which only the pre-lithiation process wasperformed and the sodium or potassium doping process was not performedhad higher initial efficiency and discharge capacity than ComparativeExample 1, but had poor life characteristics so that a life retentionrate was about 75.8%.

In Comparative Example 3, the pre-lithiation process was not performedand only the sodium and potassium doping process was performed. As aresult, SiO_(x) and Li ions reacted to produce a large amount of anirreversible phase to cause many side reactions with electrodedeterioration, and thus, initial efficiency was about 73.2% which waspoor as compared with the examples. In addition, Comparative Example 3in which the pre-lithiation process was not performed did not satisfyRelation (1), and as a result, did not obtain the sodium and potassiumdoping effect, and thus, had poor battery properties such as lifecharacteristics of a life retention rate of about 68.1%.

Comparative Example 4 in which sodium was excessively doped andComparative Example 6 in which potassium was excessively doped did notsatisfy Relation (1) with the value of Relation (1) of less than 300. Asa result, a smooth reaction between Li ions and electrons was ratherhindered and the life properties were deteriorated so that the liferetention rate was about 81.6% and 82.2%, respectively.

Comparative Example 5 in which lithium was excessively doped during thepre-lithiation process did not satisfy Relation (1), and had adifficulty in electrode manufacture due to the deteriorated slurryphysical properties. As a result, a uniform current was not secured evenwith sodium doping, so that the life properties were particularlydeteriorated with the life retention rate of about 43.1%, and dischargecapacity implement and initial efficiency were very poor. In particular,in Comparative Example 5, sodium was doped at the same amount as inExample 1, but a sodium content was less detected in the ICP analysis.This is the result from the fact that due to excessive lithium doping,an excessive amount of lithium compound surrounded the negativeelectrode active material so that sodium did not directly react with ordope in the negative electrode active material, and during the slurrypreparation, the excessive lithium compound on the surface of thenegative electrode active material was dissolved in moisture to removesodium or potassium together, so that doping was not sufficientlyperformed.

Comparative Example 7 in which sodium or potassium was excessively dopeddid not satisfy Relation (1). As a result, a smooth reaction between Liions and electrons was hindered by the excessive sodium and potassiumdoping, and thus, life characteristics was deteriorated so that the liferetention rate was about 55.4%.

The negative electrode for a lithium secondary battery according to anembodiment of the present invention may secure sufficient initialbattery efficiency and also secure a uniform current distribution on thesurface of a negative electrode active material by performing a processof single doping or co-doping a silicon-based material including aprelithiated silicon oxide with sodium or potassium, and thus, mayfurther improve life characteristics.

According to an embodiment of the present invention, in ICP analysis ofthe negative electrode active material layer, contents of elements inthe negative electrode active material layer satisfy the followingRelations (1) and (2), thereby securing sufficient initial batteryefficiency by a pre-lithiation process and also securing a uniformcurrent distribution of the surface of the negative electrode activematerial to further improve life characteristics:

300≤10⁶*A/(B²+C²)≤12.0*10⁶  (1)

800≤A≤140,000  (2)

wherein A is a Li content in ppm, B is a Na content in ppm, and C is a Kcontent in ppm, based on the total weight of the ICP-analyzed negativeelectrode active material layer.

What is claimed is:
 1. A negative electrode for a lithium secondarybattery comprising a negative electrode active material including: asilicon oxide; lithium; and sodium or potassium, wherein in ICP analysisof a negative electrode active material layer including the negativeelectrode active material, contents of elements in the negativeelectrode active material layer satisfy the following Relations (1) and(2):300≤10⁶*A/(B²+C²)≤12.0*10⁶  (1)800≤A≤140,000  (2) wherein A is a Li content in ppm, B is a Na contentin ppm, and C is a K content in ppm, based on the total weight of thenegative electrode active material layer.
 2. The negative electrode fora lithium secondary battery of claim 1, wherein in the ICP analysis ofthe negative electrode active material layer, the contents of elementsin the negative electrode active material layer further satisfy thefollowing Relations (3) and (4):50≤B≤20,000  (3)15≤C≤10,000  (4) wherein B is a Na content in ppm, and C is a K contentin ppm, based on the total weight of the negative electrode activematerial layer.
 3. The negative electrode for a lithium secondarybattery of claim 2, wherein in Relation (3), 200≤B≤20,000 is satisfied,or in Relation (4), 30≤C≤10,000 is satisfied.
 4. The negative electrodefor a lithium secondary battery of claim 1, wherein in the ICP analysisof the negative electrode active material layer, the contents ofelements in the negative electrode active material layer further satisfythe following Relations (5):25≤B+C  (5) wherein B is a Na content in ppm, and C is a K content inppm, based on the total weight of the negative electrode active materiallayer.
 5. The negative electrode for a lithium secondary battery ofclaim 1, wherein the negative electrode active material includes lithiumsilicate represented by the following Chemical Formula 1:Li_(x)Si_(y)O_(z)  [Chemical Formula 1] wherein 1≤x≤6, 1≤y≤4, and 0<z≤7.6. The negative electrode for a lithium secondary battery of claim 1,further comprising artificial graphite.
 7. The negative electrode for alithium secondary battery of claim 1, further comprising single-walledcarbon nanotubes.
 8. A method of manufacturing a negative electrode fora lithium secondary battery, the method comprising: a1) performingpre-lithiation by mixing a silicon-based material including a siliconoxide and a lithium precursor and heat treating the mixture to dope thesilicon-based material with lithium; a2) stirring a solution ordispersion including the silicon-based material doped with lithium and asodium precursor or a potassium precursor; and a3) heat treating theresultant product of the process a2, thereby preparing a negativeelectrode active material doped with sodium or potassium.
 9. The methodof manufacturing a negative electrode for a lithium secondary battery ofclaim 8, wherein in the process a1, the silicon-based material and thelithium precursor are mixed so that a Li/Si molar ratio is 0.3 to 1.2.10. The method of manufacturing a negative electrode for a lithiumsecondary battery of claim 8, wherein the stirring in the process a2 iscontinued until a Na/Si mole ratio is more than 0 and 0.05 or less or aK/Si mole ratio is more than 0 and 0.08 or less.
 11. The method ofmanufacturing a negative electrode for a lithium secondary battery ofclaim 8, wherein a stirring speed in the process a2 is 100 to 3000 rpm.12. The method of manufacturing a negative electrode for a lithiumsecondary battery of claim 8, wherein a temperature during the stirringin the process a2 is 15 to 80° C.
 13. The method of manufacturing anegative electrode for a lithium secondary battery of claim 8, whereinthe heat treating in the process a3) is performed at 200 to 1000° C. 14.The method of manufacturing a negative electrode for a lithium secondarybattery of claim 8, wherein the sodium precursor includes one of a Nametal; a Na oxide; a Na compound or Na oxide containing one or more ofF, Cl, Br, I, C, N, P, S, and H; and a Na composite oxide containing oneor more metals of Li, Ti, V, Cr, Mn, Fe, Co, and Ni, or a combinationthereof.
 15. The method of manufacturing a negative electrode for alithium secondary battery of claim 8, wherein the potassium precursorincludes one of a K metal; a K oxide; a K compound or K oxide containingone or more of F, Cl, Br, I, C, N, P, S, and H; and a K composite oxidecontaining one or more metals of Li, Ti, V, Cr, Mn, Fe, Co, and Ni, or acombination thereof.
 16. The method of manufacturing a negativeelectrode for a lithium secondary battery of claim 8, wherein thesilicon-based material is selected from the group consisting of SiO_(x)(0<x≤2); and one or more of Si, a Si-containing alloy, and a Si/Ccomposite.
 17. A method of manufacturing a negative electrode for alithium secondary battery, the method comprising: b1) stirring asolution or dispersion including a silicon-based material including asilicon oxide, a lithium precursor, and a sodium precursor or apotassium precursor; and b2) heat treating the resultant product of theprocess b1), thereby preparing a negative electrode active materialdoped with sodium or potassium.
 18. The method of manufacturing anegative electrode for a lithium secondary battery of claim 17, whereinin the process b1 the stirring of the solution or dispersion includingthe silicon-based material and the sodium precursor or the potassiumprecursor continues to obtain a Na/Si mole ratio of more than 0 and 0.05or less or a K/Si mole ratio of more than 0 and 0.08 or less.
 19. Themethod of manufacturing a negative electrode for a lithium secondarybattery of claim 17, wherein the heat treating in the process b2 isperformed at 200 to 1000° C.
 20. A lithium secondary battery comprisingthe negative electrode of claim 1.